Piezoelectric materials and magnetostrictive materials (collectively referred to below as xe2x80x9cpiezoelectric materialsxe2x80x9d) deform when an electric field or magnetic field is applied. Thus piezoelectric materials, when used as an actuator, are capable of controlling the relative position of two surfaces.
Piezoelectricity is the general term to describe the property exhibited by certain crystals of becoming electrically polarized when stress is applied to them. Quartz is a good example of a piezoelectric crystal. If stress is applied to such a crystal, it will develop an electric moment proportional to the applied stress.
This is the direct piezoelectric effect. Conversely, if it is placed in an electric field, a piezoelectric crystal changes its shape slightly. This is the inverse piezoelectric effect.
One of the most used piezoelectric materials is the aforementioned quartz. Piezoelectricity is also exhibited by ferroelectric crystals, e.g. tourmaline and Rochelle salt. These already have a spontaneous polarization, and the piezoelectric effect shows up in them as a change in this polarization. Other piezoelectric materials include certain ceramic materials and certain polymer materials. Since they are capable of controlling the relative position of two surfaces, piezoelectric materials have been used in the past as valve actuators and positional controls for microscopes. Piezoelectric materials, especially those of the ceramic type, are capable of generating a large amount of force. However, they are only capable of generating a small displacement when a large voltage is applied. In the case of piezoelectric ceramics, this displacement can be a maximum of 0.1% of the length of the material. Thus, piezoelectric materials have been used as valve actuators and positional controls for applications requiring small displacements.
Two methods of generating more displacement per unit of applied voltage include bimorph assemblies and stack assemblies. Bimorph assemblies have two piezoelectric ceramic materials bonded together and constrained by a rim at their edges, such that when a voltage is applied, one of the piezoelectric materials expands. The resulting stress causes the materials to form a dome. The displacement at the center of the dome is larger than the shrinkage or-expansion of the individual materials. However, constraining the rim of the bimorph assembly decreases the amount of available displacement. Moreover, the force generated by a bimorph assembly is significantly lower than the force that is generated by the shrinkage or expansion of the individual materials.
Stack assemblies contain multiple layers of piezoelectric materials interlaced with electrodes that are connected together. A voltage across the electrodes causes the stack to expand or contract. The displacements of the stack are equal to the sum of the displacements of the individual materials. Thus, to achieve reasonable displacement distances, a very high voltage or many layers are required. However, conventional stack actuators lose positional control due to the thermal expansion of the piezoelectric material and the material(s) on which the stack is mounted.
Due to the high strength, or stiffness, of piezoelectric material, it is capable of opening and closing against high forces, such as the force generated by a high pressure acting on a large surface area. Thus, the high strength of the piezoelectric material allows for the use of a large valve opening, which reduces the displacement or actuation necessary to open or close the valve.
With a conventional piezoelectrically actuated relay, the relay is xe2x80x9cclosedxe2x80x9d by moving a mechanical part so that two electrode components come into electrical contact. The relay is xe2x80x9copenedxe2x80x9d by moving the mechanical part so that the electrode components are no longer in electrical contact. The electrical switching point corresponds to the contact between the electrode components of the solid electrodes.
Liquid metal micro switches have been developed that use liquid metal as the switching element and the expansion of a gas when heated to actuate the switching function. The liquid metal has some advantages over other micromachined technologies, such as the ability to switch relatively high power ( approximately 100 mW) using metal-to-metal contacts without microwelding, the ability to carry this much power without overheating the switch mechanism and adversely affecting it, and the ability to latch the switching function. However, the use of a heated gas to actuate the switch has several disadvantages. It requires a relatively large amount of power to change the state of the switch, the heat generated by switching must be rejected effectively if the switch duty cycle is high, and the actuation speed is relatively slow, i.e., the maximum switching frequency is limited to several hundred Hertz.
The present invention uses a piezoelectric method to actuate liquid metal switches. The actuator of the invention uses piezoelectric elements in an extension mode rather than in a bending mode. A piezoelectric driver in accordance with the invention is a capacitive device which stores energy rather than dissipating energy. As a result, power consumption is much lower, although the required voltages to drive it may be higher. Piezoelectric pumps may be used to pull as well as push, so there is a double-acting effect not available with an actuator that is driven solely by the pushing effect of expanding gas. Reduced switching time results from use of piezoelectric switches in accordance with the invention.
The present invention uses a piezoelectric method to actuate a liquid metal relay. The method described here uses the piezoelectric element in an extension mode to cause the switch actuator to insert into a cavity in the static (i.e. nonmoving) switch contact structure. The cavity has sides and a pad on its end that are wettable by the liquid metal. The cavity is filled with liquid metal. Insertion of the switch actuator into the cavity causes the liquid metal to be displaced outward and come in contact with the contact pad on the switch actuator. The volume of liquid metal is chosen so that when the actuator returns to its rest position, the electrical contact is maintained by surface tension and by wetting of the contact pads on both the static switch contact structure and the actuator.
When the switch actuator retracts away from the static switch contact structure, the available volume for liquid metal inside the static switch contact structure increases and combination of the movement of the liquid metal into the cavity and the contact pad on the switch actuator moving away from the bulk of the liquid metal causes the liquid metal connection between the static and moving contact pads to be broken. When the switch actuator returns to its rest position, the contact remains electrically open because there is not enough liquid metal to bridge the gap without being disturbed. The switch actuator may have a coating that is wettable by the liquid metal on the part that is inserted into the liquid metal. This coating is not connected with the contact pad and exists to promote the xe2x80x9csucking backxe2x80x9d of the liquid metal when the switch actuator retracts.